The present invention relates to a longitudinal sealing device for producing a sealed seam.
In the case of horizontal tubular bag packaging machines, an endless film is formed around the products/product groups to be packaged by means of a so-called forming shoulder to form a tube, the lateral film edges being placed one on top of another and sealed to form a fin seam. For transporting the tube with the products located therein and consequently also the unrolling of the films from the film roll is carried out as a rule by way of pairs of contra-rotating rollers which convey the fin seam. At least one of the pairs of rollers is heated, as a result of which the two film regions that are located one on top of another are welded by the fin seam.
A cross-seam is then welded between two products/product groups transversely with respect to the running direction of the tube by way of a cross-sealing station and said cross-seam is separated by way of a blade. This creates individual so-called tubular bags. By way of said tubular bags, the product is protected on the one hand against mechanical influences, it also being possible to achieve protection against ingress of moisture and/or of gases/oxygen.
Solutions which convey and seal the longitudinal sealed seam by means of three pairs of rollers which are located one behind another in the conveying direction of the tube are known from the prior art. In this case, as a rule, the first roller pair is cold and is provided exclusively for joining together the parts of the film to be welded and for transporting the tube. The film parts to be welded are also designated as fins. A fin is to be understood as film edge regions which are placed one on top of the other by way of their inside surface. This is referred to in technical jargon as a fin seam. The second roller pair is heated and the third roller pair can also be heated but can also be cold. In this case, the job of the third roller pair is to seal the film region, which has been melted by the second roller pair, by means of pressure and additionally, as an option, heat. It consequently has to be arranged on the same plane or at the same height as the second pair of rollers. At the same time, the tube is conveyed by way of said pair of rollers.
Good results can be achieved with the three rollers that are arranged one behind another when using so-called multiple layer or multi-layer films. Said films typically comprise a mechanically stable and non-heat-sensitive carrier layer and at least one or several sealing layers. The sealing layers melt on exposure to heat. On account of the carrier layer, said films are stable against tensile forces produced by their layer structure. Conveying the tube by said melted regions functions well as the carrier layer provides the necessary stability.
However, there are applications where mono-films are used. In the case of such mono-films there is no mechanically stable carrier layer. Low-priced, thinner hot-sealable films, which lose stability very rapidly as a result of being heated up, are also used more and more frequently. When said mono-films are conveyed using the above-mentioned devices with the three pairs of rollers, it has been ascertained that the weld point is negatively influenced by means of the roller which follows after the heated roller. Tensile forces occur in the material which weaken the weld. In addition, it has been ascertained that the film could become contorted, which is also very disadvantageous because it then has a negative impact on the tightness of the weld.
Said negative factors occur increasingly, in particular in the case of higher film speeds.